Taz Posted December 23, 2014 Share Posted December 23, 2014 my mc 30 started an intermittent sound. Every few songs it makes me think of what an arc welder would sound like hooked to the Khorns. Just the one for the right channel. Reversed speakers at controler. no help. Backed off on the bass. no help. Now I'm wondering if I should start swapping tubes from one side to the other. But willing to listen to any words of wisdom. Happy Holidays to each of you, whatever your Holiday is this time of year. Going to try another receiver. But I had no problem with the SS unit I was using prior. Quote Link to comment Share on other sites More sharing options...
Audible Nectar Posted December 23, 2014 Share Posted December 23, 2014 What do you know of the history of these particular amplifiers? Have they been serviced or rebuilt? My original pair of MC30s exhibited similar issues - the problem was not solved until they were fully rebuilt. You might try swapping the tubes from the left amp to the right amp to see if the issue "travels". Eliminate all other possibilities first - swap speaker leads then see if the issue "travels", then swap preamp outputs to the amp to see if the issue travels. Also make sure the leads from amp to speaker are solid. BTW - what is "the controller"? Are you using a speaker switcher of some kind? Quote Link to comment Share on other sites More sharing options...
John Warren Posted December 23, 2014 Share Posted December 23, 2014 Check resistance values of: R25, R28, R27, R29, R20 and R23. Note that R20 and R23 should be within about 1% of each other. Quote Link to comment Share on other sites More sharing options...
Taz Posted December 23, 2014 Author Share Posted December 23, 2014 "the controller" is a Lafayette KT-600. If I'm not mistaken, I believe you at one time stated you knew the guy that designed it. History. got them from my neighbor. Refused to even plug them in until they were serviced. They have not been rebuilt. Or at least I don't believe they were. Cost was $1,000 to have them gone through and some tubes replaced. Suggestions on who to have look at if I do send them in. Live in Roseburg Or. The business that I used to hand carry my gear in to be worked on is no longer in business. Seems we're all getting older and retiring. Quote Link to comment Share on other sites More sharing options...
Taz Posted December 23, 2014 Author Share Posted December 23, 2014 Check resistance values of: R25, R28, R27, R29, R20 and R23. Note that R20 and R23 should be within about 1% of each other. Huh! Never did much under the hood work on stereos. So I should go into the bottom of the mc 30 to check resistance. But have no idea how to find R25, R28 etc. I do have multi meter though. somewhere I do have an assembly book for mc 30 kit. Quote Link to comment Share on other sites More sharing options...
John Warren Posted December 23, 2014 Share Posted December 23, 2014 (edited) service manual attached MC30 Service S.pdf Edited December 23, 2014 by John Warren Quote Link to comment Share on other sites More sharing options...
Taz Posted December 25, 2014 Author Share Posted December 25, 2014 Good luck. If you are not feeling confident, let someone with a bit more experience do this for you. You don't want to hurt yourself over this. The easier stuff like wiggling things I'm reasonably comfortable with. Don't want to turn out like Thebes. Couldn't help myself on that one. Actually I think I might like to be like Thebes. But am wondering what it might cost for refurbish on the 2 mc30's and the Lafayette KT-600. Know it is not possible to get firm quote without sending them in. PM is good for guessing cost if anyone would rather tell me that way. I don't know, but it seems to me the Lafayette and mc-30's should blow away The Fisher 400. That has not been the case all the time though. I'm having some of the old stuff refurbished. Since this set up is keeper's I might as well think about getting them done also. While I still can. Dr. ordered early retirement. Not going hungry. Bills are being paid. So in that respect I'm doing pretty good. I've got a Pelican case for shipping stereo's. It's in Montana at present with a Marantz 2500. Next will be a 2325. Then I could ship the mc30's and the Lafayette KT-600. Quote Link to comment Share on other sites More sharing options...
Audible Nectar Posted December 25, 2014 Share Posted December 25, 2014 Talk to Craig (NOSValves). He knows what to do with those. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted December 26, 2014 Moderators Share Posted December 26, 2014 Talk to Craig (NOSValves). He knows what to do with those. I would strongly second that advice. Call Craig at NosValves and he can give you a good idea of what to expect. Quote Link to comment Share on other sites More sharing options...
Marvel Posted December 26, 2014 Share Posted December 26, 2014 Craig would be good, and Terry DeWick would be the other superb Mac repair guy. http://www.mcintoshaudio.com/ Quote Link to comment Share on other sites More sharing options...
Coytee Posted December 26, 2014 Share Posted December 26, 2014 Got to give a nod to DeWick. He's probably the gold standard. I once called McIntosh with a tube question about my MC-2102. The guy asked "where did you say you were?" (Knoxville) "Do you know Terry DeWick?" Yep "I think you would be better served asking your question to him" Ok, so wait a minute.....this is a McIntosh tube amp.... McIntosh designed & built it. I'm talking to McIntosh technical help guy.....and you (who built the thing) are referring me to go talk to Terry DeWick? Yep Hard to beat a recommendation like that other than Terry has a big backload. I'd wager that Craig could get to something & fix it like 54,956 times before Terry might have the time to get to it once. Quote Link to comment Share on other sites More sharing options...
Taz Posted December 30, 2014 Author Share Posted December 30, 2014 (edited) Isolated the problem to the Preamp. Have been in contact with Terry D. Edited December 30, 2014 by Taz Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 30, 2014 Share Posted December 30, 2014 Terry Dewick is about the only competitor I would comfortably recommend. I will advise you to request he 'completely" rebuild the piece rather then just fix what is broke. Spend the money and do it all your long term enjoyment will be much better. 1 Quote Link to comment Share on other sites More sharing options...
John Warren Posted December 31, 2014 Share Posted December 31, 2014 (edited) Isolated the problem to the Preamp. Have been in contact with Terry D. Good, Mr. Dewick will also provide performance data after the job is complete. For example, I've attached a %THD graph of an amp I'm in the process of over-hauling (a McIntosh solid state amp, I don't do tubes). The VAS (a single NPN transistor) on the left channel power output boards had issues and failed (c-e junction was shorted). I replaced both the discrete device and the emitter degeneration resistor. While I was at it and because the parts are relatively straightforward to obtain thru Mouser or Digi-key, I replaced the differential input pair, the complimentary push-pull drivers, the electrolytic caps with Nichicon types and the pF ceramic discs. The green and dark blue curves are the repaired board (replicates), the light blue and gold are from the right channel board that appeared to be ok for the moment. So, the plot demonstrates that the board is functioning (pretty much) as what one would expect. The max power in this set of plots is only about 12W (the amp is capable much more). There are other things that require attention so need to go in baby steps here. In the old days. DOB provided plots of distortion and such at the McIntosh clinics. I remember walking out of the Stereo Shops in Newton Heights, MA (owned by Gordon Gow's wife Palma, btw) around 1976 with performance plots of my MC2105 feeling all "warm and fuzzy" that I was well below %THD specification max limits. I recall sitting patiently aside DOB while he ran the amp thru all it's performance tests. The place was dead, it was snowing pretty hard so he took a lot of time and even pulled a coupled of boards and replaced what can only be presumed to be known, "problem" components. Edited January 12, 2015 by John Warren Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 1, 2015 Share Posted January 1, 2015 (edited) Here's the distortion plot to about 70W (for which the amp is rated). Distortion is now quite low to rated power. Not bad for an amp that's almost 40 years old! Edited January 1, 2015 by John Warren Quote Link to comment Share on other sites More sharing options...
Taz Posted January 1, 2015 Author Share Posted January 1, 2015 Here's the distortion plot to about 70W (for which the amp is rated). Distortion is now quite low to rated power. Not bad for an amp that's almost 40 years old! power_amp_%THD_23V.jpg 1959 and 1960. Almost 50 yrs. Or I'm confused again. Quote Link to comment Share on other sites More sharing options...
John Warren Posted January 2, 2015 Share Posted January 2, 2015 The plot above is an example of what you should get from the supplier you select to fix your amp, it's not an MC30. I did own a pair of MC30 amps, but sold them. Also had an original Williamson amp which I tossed into a trash bin when I cleaned out my apartment in Cambridge, MA (circa 1982). Quote Link to comment Share on other sites More sharing options...
Gilbert Posted January 5, 2015 Share Posted January 5, 2015 (edited) Suggestions on who to have look at if I do send them in. That's a no-brainer. Send them to Craig, aka NOSVALVES. He's rebuilt 4 MC60's for me, still going strong, never an issue. Another piece of equipment I had purchased, is a C22 preamp that Terry Dewick had supposedly gone through. I purchased the pre specifically because Terry had recently worked on it. I shelled out some serious cash for that preamp. The seller even sent me the original invoice from Terry Dewick. That damn thing never worked properly, and was probably the reason the guy unloaded it. Anyway, because of the quality work Craig did on my MC60's, I sent him the C22 for a complete rebuild. This was quite a few years ago, and it's still going strong. Not one problem since, not with my MC60's or my C22.. Not trying to drum-up business for Craig, don't think I need to do that. The fact is, I think he's the "Smokey Yunick" of vintage audio equipment. Give him anything, he'll make it better. Edited January 5, 2015 by Gilbert 1 Quote Link to comment Share on other sites More sharing options...
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